mfd: ab8500-core: Return zero in get_register_interruptible()

This function is supposed to return zero on success or negative error
codes on error.  Unfortunately, there is a bug so it sometimes returns
non-zero, positive numbers on success.

I noticed this bug during review and I can't test it.  It does appear
that the return is sometimes propogated back to _regmap_read() where all
non-zero returns are treated as failure so this may affect run time.

Fixes: 47c16975 ("mfd: Align ab8500 with the abx500 interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>